linux-kernel - Re: [PATCH v6] iomap: support tail packing inline read

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Message-ID: <YPrms0fWPwEZGNAL@casper.infradead.org>
Date:   Fri, 23 Jul 2021 16:56:35 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@....de>, linux-erofs@...ts.ozlabs.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Andreas Gruenbacher <andreas.gruenbacher@...il.com>
Subject: Re: [PATCH v6] iomap: support tail packing inline read

On Fri, Jul 23, 2021 at 11:23:38PM +0800, Gao Xiang wrote:
> Hi Matthew,
> 
> On Fri, Jul 23, 2021 at 04:05:29PM +0100, Matthew Wilcox wrote:
> > On Thu, Jul 22, 2021 at 07:39:47AM +0200, Christoph Hellwig wrote:
> > > @@ -675,7 +676,7 @@ static size_t iomap_write_end_inline(struct inode *inode, struct page *page,
> > >  
> > >  	flush_dcache_page(page);
> > >  	addr = kmap_atomic(page);
> > > -	memcpy(iomap->inline_data + pos, addr + pos, copied);
> > > +	memcpy(iomap_inline_buf(iomap, pos), addr + pos, copied);
> > 
> > This is wrong; pos can be > PAGE_SIZE, so this needs to be
> > addr + offset_in_page(pos).
> 
> Yeah, thanks for pointing out. It seems so, since EROFS cannot test
> such write path, previously it was disabled explicitly. I could
> update it in the next version as above.

We're also missing a call to __set_page_dirty_nobuffers().  This
matters to nobody right now -- erofs is read-only and gfs2 only
supports inline data in the inode.  I presume what is happening
for gfs2 is that at inode writeback time, it copies the ~60 bytes
from the page cache into the inode and then schedules the inode
for writeback.

But logically, we should mark the page as dirty.  It'll be marked
as dirty by ->mkwrite, should the page be mmaped, so gfs2 must
already cope with a dirty page for inline data.